In the future, optical input/output (I/O) will be used in computer systems to transmit data between system components. Optical I/O is able to attain higher system bandwidth with lower electromagnetic interference than conventional copper I/O methods. Optical I/O can achieve a higher bandwidth-distance product than electrical I/O, allowing systems to operate at high bandwidth across physically longer links, such as a back plane interconnect. In order to implement optical I/O, radiant light energy is coupled to a waveguide such as an optical fiber from an optoelectronic device such as a laser at a transmitting position in the link. At a receiving position of the link, a photo detector is coupled to the waveguide, to receive the propagating optical communications signal. Additional circuitry, typically electronic circuitry, is provided at the transmitting as well as the receiving positions, to modulate and demodulate the optical signal with the information or payload data that is being transferred by the link. Currently, communication systems such as switches, routers, and other packet and time division multiplexed (TDM) processing devices use optical communication links to great advantage.
In the quest to increase the bandwidth of an optical link, several topologies have been proposed and implemented. These topologies increase the number of waveguides operating in parallel, launch a multitude of different wavelengths simultaneously in the same waveguide, and/or operate a waveguide in full duplex. For example, there is the basic, multi-wavelength optical link in which a single waveguide is driven in just one direction by an optical signal that has the payload carried by multiple (different) wavelengths. At an upstream unit, several transmitters transform the electrical data into different wavelength optical signals, multiplex or combine them into a single waveguide and then transmit through this single waveguide in a downstream direction. At a downstream unit, a demultiplexer does the reverse-it separates the wavelengths and converts the different wavelength signals into several electrical signals. In that case, data is transferred over the single waveguide in just one direction. To transfer data in the opposite direction, a second, identical link may be provided (with a separate waveguide).
In another type of optical link, a single waveguide is used bi-directionally, to transfer data in both directions. At a first end of the waveguide, an optical transmitter launches a signal (containing data to be transferred), at a single wavelength λ1. At a second end of the waveguide, an optical receiver tuned to λ1 will detect that data. Also, at the second end, there is an optical transmitter that is transmitting data in the direction of the first end, using λ2. This in turn is detected at the first end by an optical receiver tuned to λ2. Chromatic filters are used to spectrally separate the counter-propagating signals that have different wavelengths, at each end of the waveguide. Such a link can transfer data simultaneously in both directions, over the single waveguide.
In a further attempt to increase bandwidth, parallel optical links have been proposed that have multiple waveguides in each direction of propagation. In other words, instead of having a single waveguide to transfer data in each direction, there are multiple waveguides where each can transfer data simultaneously in parallel, in the same direction. For two way communications, each end point has a parallel optical transmitter and a parallel optical receiver. In this case, each waveguide is operated unidirectionally.